In the search for ever-faster solid state devices, many types of physical effects have been examined in the hopes that they would lead to faster devices or devices with more desirable characteristics. One of the more interesting physical phenomena so examined is particle tunneling through an energy barrier.
Perhaps the first such device using this effect was the "Tunnel Emission Amplifier, " which was proposed by C. A. Mead in 1960. See, for example, Proceedings of the IRE, pp. 359-361, Mar. 1960. The proposed device had a metal-insulator-metal-insulator-metal structure with the current through the first metal-insulator-metal structure occurring primarily by tunneling. Another tunneling device was proposed by Kisaki in Proceedings of the IEEE, pp. 1053 -1054, July 1973. The structure, which was termed a "tunnel transistor", had a metal-insulator-semiconductor structure with carriers tunneling from the metal emitter electrode through an insulator layer into the base. Tunneling in more complicated structures has also been studied. For example, the transport properties of carriers in a finite superlattice were also studied theoretically, with tunneling considered, in Applied Physics Letters, 22, pp. 562-564, June 1, 1973. Tunneling in this structure is of interest because of the possibility of obtaining negative differential conductivity.
Perhaps a still more interesting tunneling phenomenon is termed "resonant tunneling". Structures exhibiting resonant tunneling have two or more energy barrier layers surrounding one or more potential well layers. Resonant tunneling occurs when the carrier goes through an energy eigenstate of the well. Enhanced tunneling probabilities may be obtained as well as characteristics such as negative differential resistance. A resonant tunneling structure was studied by Chang, Esaki, and Tsu in Applied Physics Letters, 24 pp. 593-595, June 15, 1974, for the case of a double barrier. The resonance and current maxima occur when the applied voltages to the barrier layers are such that the Fermi energy at the electrodes is equal to that of one of the states in the potential well.
Earlier work by Davis et al, Journal of Applied Physics, 34 pp. 864-866, Apr., 1963, discussed a resonant tunneling triode having a metal-insulator-metal-insulator-metal structure. The device was a unipolar, majority carrier device and was similar to the device proposed by Mead in Journal of Applied Physics, 32, pp. 646-652, 1961.
A device termed the "tunnel triode" was proposed in Applied Physics Letters, 31,, pp. 687-689, Nov. 15, 1977. This article clearly points out one problem with may of the previously proposed tunnel devices, namely, that the region through which the carriers tunnel is an insulator, and it is therefore difficult to attach an electrode to that region. This, of course, makes three-terminal devices difficult to fabricate. The tunnel triode avoids this problem by using a structure having staggered heterojunctions with an energy bandgap structure such that carriers of one type tunnel through a barrier formed by the base region in which carriers of the other type are confined. The presence of the electrons in the base ensures that the base will be electrically conductive.
In spite of the effort expended, a review of the devices discussed shows that tunneling devices have not, in practice, lived up to their theoretical expectations. It has recently been realized that the physics of resonant tunneling contain physical effects which must be more carefully considered if device operation is to be optimized. To better understand these effects, it is useful to consider an analogy with a Fabry-Perot resonator. If the reflectivities of the mirrors in a Fabry-Perot resonator differ significantly from each other, the transmission through the cavity at the resonant frequencies decreases significantly below unity. An analogous situation arises in resonant tunneling through a double barrier with equal barrier heights and thicknesses when the application of an electric field produces a difference between the transmission coefficients of the two barriers. This breaks the intrinsic symmetry of the double barrier and can lead to an order of magnitude decrease in the tunneling current at resonance as the transmission probability for one barrier becomes significantly greater than the transmission probability for the other barrier. For example, the peak-to-valley ratio for a negative differential resistance device may be significantly less than expected. The effects are discussed in detail in, for example, Physical Review B, 29, pp. 1970-1981, Feb. 15, 1984.
This problem may be overcome by making one of the barriers thicker or higher than the other one. However, this is not a totally satisfactory solution, as the quality of the transmission coefficients and overall unity transmission is achieved for only one of the well resonances. The transmission for the remaining resonances continues to be significantly less than unity. For some applications, it is desirable that the total transmission be unity for several resonances.